[RISCV] Add option aliases: -mcmodel=medany and -mcmodel=medlow

RISC-V GCC use -mcmodel=medany and -mcmodel=medlow, but LLVM use
-mcmodel=small and -mcmodel=medium.

Add those two option aliases for provide same user interface between
GCC and LLVM.

Reviewed By: lenary

Differential Revision: https://reviews.llvm.org/D67066

llvm-svn: 372080